Semiconductor device

ABSTRACT

According to one embodiment, a semiconductor device includes an insulating substrate, a first semiconductor layer formed of silicon and positioned above the insulating substrate, a second semiconductor layer formed of a metal oxide and positioned above the first semiconductor layer, a first insulating film formed of a silicon nitride and positioned between the first semiconductor layer and the second semiconductor layer, and a block layer positioned between the first semiconductor film and the second semiconductor layer, the block layer hydrogen diffusion of which is lower than that of the first insulating film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2016-006123, filed Jan. 15, 2016, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a semiconductor device.

BACKGROUND

Thin film transistors show various properties depending on materialsused in a semiconductor layer therein. For example, if a low temperaturepolysilicon semiconductor is used as the semiconductor layer, a thinfilm transistor of good reliability can be achieved. Furthermore, if aoxide semiconductor is used as the semiconductor layer, a thin filmtransistor of a small off current can be achieved. If a thin filmtransistor including a polysilicon semiconductor layer and a thin filmtransistor including an oxide semiconductor layer are formed on the samesubstrate, the oxide semiconductor layer may become low resistivebecause of hydrogen diffusion from the polysilicon semiconductor layerto the oxide semiconductor layer, and a desired property may not beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the structure of a semiconductordevice of an embodiment.

FIG. 2 is a cross-sectional view of a variation in which a block layeris patterned.

FIG. 3 is a cross-sectional view of a variation in which the block layeris disposed in a different position.

FIG. 4 is a cross-sectional view of a variation in which the block layeris patterned.

FIG. 5 is a cross-sectional view of a variation in which a thin filmtransistor TR2 is of top gate structure.

FIG. 6 is a cross-sectional view of a variation in which the block layeris patterned based on the variation of FIG. 5.

FIG. 7 is a cross-sectional view of a variation in which the block layeris disposed in a different position based on the variation of FIG. 5.

FIG. 8 is a cross-sectional view of a variation in which the block layeris patterned based on the variation of FIG. 5.

FIG. 9 shows a display device including the semiconductor device of theembodiment.

DETAILED DESCRIPTION

In general, according to one embodiment, a semiconductor device includesan insulating substrate, a first semiconductor layer formed of siliconand positioned above the insulating substrate, a second semiconductorlayer formed of a metal oxide and positioned above the firstsemiconductor layer, a first insulating film formed of a silicon nitrideand positioned between the first semiconductor layer and the secondsemiconductor layer, and a block layer positioned between the firstsemiconductor film and the second semiconductor layer, the block layerhydrogen diffusion of which is lower than that of the first insulatingfilm.

Embodiments will be described hereinafter with reference to theaccompanying drawings. Incidentally, the disclosure is merely anexample, and proper changes within the spirit of the invention, whichare easily conceivable by a skilled person, are included in the scope ofthe invention as a matter of course. In addition, in some cases, inorder to make the description clearer, the widths, thicknesses, shapes,etc. of the respective parts are schematically illustrated in thedrawings, compared to the actual modes. However, the schematicillustration is merely an example, and adds no restrictions to theinterpretation of the invention. Besides, in the specification anddrawings, the structural elements having functions, which are identicalor similar to the functions of the structural elements described inconnection with preceding drawings, are denoted by like referencenumerals, and an overlapping detailed description is omitted unlessnecessary.

Note that, in the description of the present application, above (or up)corresponds to the direction of the arrow of the direction Z, and below(or down) corresponds to the opposite direction of the arrow of thedirection Z.

FIG. 1 is a cross-sectional view of the structure of a semiconductordevice of an embodiment. The semiconductor device 1 depicted is a thinfilm transistor (TFT) substrate including a plurality of thin filmtransistors TRl and TR2.

The semiconductor device 1 includes, for example, an insulatingsubstrate 10, undercoat layer UC, semiconductor layer SC1, insulatingfilm 11, gate electrode ML1, gate electrode ML2, insulating film 12,insulating film 13, block layer HB, semiconductor layer SC2, insulatingfilm 14, and insulating film 15. Note that the insulating films 11 to 15may be referred to as first insulating film, second insulating film,third insulating film, . . . , and/or interlayer insulating film. Thisincludes that the insulating film 11 is referred to as interlayerinsulating film, insulating film 12 is referred to as first insulatingfilm, insulating film 13 is referred to as second insulating film, andfourth insulating film 14 is referred to as interlayer insulating film.Furthermore, the semiconductor layer SC1 may be referred to as firstsemiconductor layer and the semiconductor layer SC2 may be referred toas second semiconductor layer.

The insulating substrate 10 is formed of, for example, alight-transmissive glass substrate or light-transmissive resinsubstrate. An insulating undercoat layer UC is disposed on theinsulating substrate 10. The undercoat layer UC may be a monolayer or amultilayer, and the multilayer includes, for example, a silicon nitridelayer and a silicon oxide layer.

The semiconductor layer SC1 is disposed above the insulating substrate10. In the example depicted, the semiconductor layer SC1 is formed onthe undercoat layer UC. The semiconductor layer SC1 is disposed betweenthe insulating substrate 10 and the block layer HB. The semiconductorlayer SC1 is formed of a silicon semiconductor, and in the exampledepicted, is formed of a polycrystalline silicon (polysilicon). Thesemiconductor layer SC1 includes a high resistance area SCc and lowresistance areas SCa and SCb resistance of which is lower than that ofthe high resistance area SCc. The low resistance areas SCa and SCb arepositioned at the ends of the high resistance area SCc. The lowresistance areas SCa and SCb contain ionic impurities concentratedhigher than that of the high resistance area SCc.

The insulating film 11 covers the semiconductor layer SC1. In theexample depicted, the insulating film 11 is disposed above the undercoatlayer UC. For example, the insulating film 11 is formed of a siliconoxide.

The gate electrode ML1 is disposed on the insulating film 11 and isopposed to the semiconductor layer SC1 with the insulating film 11interposed therebetween. The gate electrode ML1 is opposed to the highresistance area SCc of the semiconductor layer SC1. The gate electrodeML2 is disposed above the insulating film 11 and is apart from the gateelectrode ML1. The gate electrodes ML1 and ML2 each include a metallayer of good conductivity. In the example depicted, the gate electrodesML1 and ML2 are formed on the same layer, and thus, they can bemanufactured at the same time with the same material.

The insulating film 12 is disposed between the semiconductor layer SC1and the semiconductor layer SC2. The insulating film 12 is disposed onthe insulating film 11 to cover the gate electrodes ML1 and ML2. Theinsulating film 12 is formed of an insulative material containinghydrogen therein. In this example, the insulating film 12 is formed of asilicon nitride.

The insulating film 13 is disposed above the insulating film 12. In theexample depicted, the insulating film 13 is disposed between theinsulating film 12 and the block layer HB, and contacts the lowersurface HBa of the block layer HB which will be described later. Theinsulating film 13 is preferably formed of a material which can releaseoxygen in a high temperature such as a silicon oxide.

The block layer HB is disposed between the insulating film 12 and thesemiconductor layer SC2. The block layer HB includes the lower surfaceHBa opposed to the insulating substrate 10 and the upper surface HBbdisposed in the opposite side of the lower surface HBa. In the exampledepicted, the block layer HB is disposed on the insulating film 13. Inthe example depicted, the block layer HB continuously extends to aposition opposed to the semiconductor layer SC1 and a position opposedto the semiconductor layer SC2. The block layer HB exerts hydrogendiffusion lower than the insulating film 12 and the insulating film 13.Therefore, the block layer HE can suppress diffusion of hydrogen fromone surface of the block layer HB to the other surface thereof. In theexample depicted, the block layer HB can suppress the hydrogen diffusionfrom the insulating films 11, 12, and 13, undercoat layer UC, andsemiconductor layer SC1 positioned in the lower surface HBa side to thesemiconductor layer SC2 positioned in the upper surface HBb side.

The block layer HB may have an even thickness or an uneven thickness. Inthe example depicted, the block layer HB has steps in the upper surfaceHBb in positions opposed to outer edges of electrodes ML3 a and ML3 bwhich will be described later, and thus, the thickness of the areaopposed to the semiconductor layer SC2 is greater than the thickness ofthe area opposed to the semiconductor layer SC1. The block layer HB ispreferably formed of an insulating material in order to suppress theformation of an unnecessary capacitance with a conductive member such asgate electrode ML1 or semiconductor layer SC1 and to prevent a short ofa channel layer when the upper surface HBb of the block layer HB and thelower surface SC2 a of the semiconductor layer SC2 contact. The blocklayer HB is formed of a metal oxide such as AlO_(x), TiO_(x), ZrO_(x),TaO_(x), or HfO_(x), or SiN_(x) or SiON which contains SiF₄ as its basematerial and low hydrogen. In consideration of hydrogen blockingperformance and light transmissivity, the block layer HB is preferablyformed of aluminum oxide (AlO_(x)).

The semiconductor layer SC2 is disposed above the semiconductor layerSC1 than is the insulating substrate 10. The semiconductor layer SC2 hasthe lower surface SC2 a to be opposed to the insulating substrate 10. Inthe example depicted, the semiconductor layer SC2 is disposed on theblock layer HB and contacts the upper surface HBb of the block layer HB.The semiconductor layer SC2 is opposed to the gate electrode ML2. Thesemiconductor layer SC2 is formed of a semiconductor of metal oxide. Inconsideration of semiconductor performance, the metal oxide in thesemiconductor layer SC2 preferably contains at least one of indiu,gallium, zinc, and tin.

One end of the semiconductor layer SC2 contacts the electrode ML3 a andthe other end contacts the electrode ML3 b. In the example depicted, theelectrodes ML3 a and ML3 b extend to the outside of the semiconductorlayer SC2 to be positioned above the block layer HB.

The insulating film 14 is disposed on the block layer HB and covers thesemiconductor layer SC2 and the electrodes ML3 a and ML3 b. Theinsulating film 14 is formed of, for example, silicon oxide and isformed thicker than the insulating films 11, 12, and 13.

The insulating film 15 is disposed on the insulating film 14. Theinsulating film 15 is formed of, for example, silicon nitride. Note thatthe insulating film 15 preferably exerts a high vapor barrier in orderto prevent moisture from entering from the above.

Terminals T1 a, T1 b, T2 a, and T2 c are disposed on the insulating film15. Terminals T1 a and T1 b are each formed to pass through theinsulating films 11, 12, 13, 14, and 15 and block layer HB, and areelectrically connected to low resistance areas SCa and SCb of thesemiconductor layer SC1. Terminal T2 a is formed to pass through theinsulating films 14 and 15 and is electrically connected to theelectrode ML3 a. Terminal T2 c is formed to pass through the insulatingfilms 12, 13, 14, and 15 and block layer HB and is electricallyconnected to the gate electrode ML2. Terminals T1 a, T1 b, T2 a, and T2c are electrically connected to lines or the like which are not depictedin the figure.

In the example depicted, the thin film transistor TR1 is a top gate thinfilm transistor in which the gate electrode ML1 is disposed above thesemiconductor layer SC1. Furthermore, a thin film transistor TR2 is abottom gate thin film transistor in which the gate electrode ML2 isdisposed below the semiconductor layer SC2. Note that the structure ofeach of the thin film transistors TR1 and TR2 is not limited to theabove example, and the thin film transistor TR1 may be of bottom gatetype and the thin film transistor TR2 may be of top gate type.

As above, in the present embodiment, the semiconductor device 1 includesthe insulating substrate 10, silicon semiconductor layer SC1, metaloxide semiconductor layer SC2, silicon nitride insulating film 12between the semiconductor layer SC1 and the semiconductor layer SC2, andblock layer HB between the insulating film 12 and the semiconductorlayer SC2. Thus, the hydrogen diffusion from the semiconductor layer SC1and the insulating film 12 to the semiconductor layer SC2. That is,reduction of the semiconductor layer SC2 by hydrogen can be suppressedand degradation of reliability of the thin film transistor TR2 can besuppressed, too.

The semiconductor layer SC1 is disposed between the insulating substrate10 and the block layer HB. Thus, in a manufacturing process where eachmember is laminated on the insulating substrate 10, the formation andactivation of the semiconductor layer SC1 can be performed beforeforming the semiconductor layer SC2. Thereby, a change in components ofthe semiconductor layer SC2 caused by heat applied in the activationprocess of the semiconductor layer SC1 can be prevented. Thus,degradation of reliability of the thin film transistor TR2 can besuppressed.

The semiconductor layer SC2 contacts the block layer HB and hydrogenentering the semiconductor layer SC1 from the lower surface SC2 a sidecan further be blocked. Furthermore, the block layer HB continuouslyextends to a position opposed to the semiconductor layer SC1 and aposition opposed to the semiconductor layer SC2, and thus, the hydrogendiffusion to the above structure from the insulating film 12 and thesemiconductor layer SC1 can be suppressed more effectively.

The semiconductor device 1 further includes the gate electrodes ML1 andML2. The gate electrodes ML1 and ML2 are disposed on the same layer(insulating film 11) and are formed of the same material. Thus, the gateelectrodes ML1 and ML2 can be formed in the same process. That is, thesemiconductor device 1 can be manufactured through lesser processes withlesser costs.

The block layer HB is formed of, for example, an aluminum oxide, andthus, the block layer HB can be formed by using aluminum formation rateof which is higher than aluminum oxide and oxidizing the aluminumthrough an annealing process. Therefore, a time used for manufacturingthe semiconductor device 1 can be reduced. Furthermore, if the blocklayer HB of aluminum oxide contacts the semiconductor layer SC2,aluminum is diffused from the block layer HB to the semiconductor layerSC2 and carriers are supplied, and thus, the mobility of thesemiconductor layer SC2 can be improved.

Now, variations of the present embodiment will be explained withreference to FIGS. 2 to 4. Note that the advantages obtained in theabove embodiment can be achieved in these variations.

FIG. 2 is a cross-sectional view of a variation in which the block layeris patterned.

In the variation, the block layer HB is patterned to have an islandshape. In this respect, the variation differs from the example of FIG.1.

In the example depicted, the block layer HB are formed as islands in thearea opposed to the entire surface of the semiconductor layer SC2 andthe area opposed to the electrodes ML3 a and ML3 b. The block layer HBis not formed in the other area, and is not opposed to the semiconductorlayer SC1, for example. The block layer HB depicted can be patternedusing the semiconductor layer SC2 and the electrodes ML3 a and ML3 b asa mask. Note that, if the block layer HB is formed in an island shape,the area of the block layer HB is preferably formed larger than that ofthe semiconductor layer SC2 in order to block hydrogen entering thesemiconductor layer SC2.

In this variation, the block layer HB is patterned as islands, and thenumber of interfaces in the areas between thin film transistors can bereduced. Thereby, the transmissivity of the areas between the thin filmtransistors can be improved in this variation. If the semiconductordevice 1 of the present embodiment is applied to a transmissive displaydevice, the areas between thin film transistors correspond to openingsthrough which light passes, and thus, a display device of highluminosity can be achieved.

FIG. 3 is a cross-sectional view of a variation in which the block layeris disposed in a different position.

The insulating film 13 is disposed between the block layer HB and thesemiconductor layer SC2 in this variation, and in this respect, thevariation differs from the example of FIG. 1. In the example depicted,the block layer HB is disposed on the insulating film 12 and theinsulating film 13 is disposed on the block layer HB, and thesemiconductor layer SC2 is disposed on the insulating film 13. That is,the insulating film 13 contacts the upper surface HBb of the block layerHB and the lower surface SC2 a of the semiconductor layer SC2.

In this variation, the hydrogen diffusion form the insulating film 12 tothe insulating film 13 can be suppressed. That is, the hydrogendiffusion can be suppressed in a position closer to the hydrogensupplier.

FIG. 4 is a cross-sectional view of a variation in which the block layeris patterned.

In this variation, the block layer HB is patterned as islands, and inthis respect, the variation differs from the variation of FIG. 3. Theadvantages obtained in the variation of FIG. 2 can be achieved in thisvariation.

FIG. 5 is a cross-sectional view of a variation in which the thin filmtransistor TR2 is of top gate structure.

In this variation, a gate electrode ML4 is disposed to be opposed to thesemiconductor layer SC2, and in this respect, this variation differsfrom the example of FIG. 1.

The gate electrode ML4 is disposed above the semiconductor layer SC2,and in the example depicted, the gate electrode ML4 is disposed betweenthe insulating film 14 and the insulating film 15. In the exampledepicted, a light shielding layer SH is disposed below the semiconductorlayer SC2 to be opposed to the semiconductor layer SC2. The lightshielding layer SH and the gate electrode ML1 are disposed on the samelayer (insulating film 11), and can be formed through the samemanufacturing process with the same material used for the gate electrodeML1. The light shielding layer SH can block the light entering into thesemiconductor layer SC2 if the light from the below is incident on thesemiconductor device 1. Furthermore, in the example depicted, theinsulating film 14 is patterned as with the gate electrode ML4; however,it may be formed on the entire surface as with the insulating film 15.

In this variation, the semiconductor device 1 can suppress thedegradation of performance of the thin film transistor TR2 by lightleakage current.

Now, variations of the present embodiment will be explained withreference to FIGS. 6 to 8. Note that the same advantages obtained in theabove embodiment can be achieved in these variations.

FIG. 6 is a cross-sectional view of a variation in which the block layeris patterned.

In this variation, the block layer HB is patterned as islands, and inthis respect, the variation differs from the variation of FIG. 5.

In the variation, the block layer HB is formed as islands in the areaopposed to the entire surface of the semiconductor layer SC2. The blocklayer HB is not formed in the other area, and is not opposed to, forexample, the semiconductor layer SC1. The block layer HB depicted ispatterned using the semiconductor layer SC2 as a mask. Note that, if theblock layer HB is formed in an island shape, the area of the block layerHB is preferably formed at least greater than the area of thesemiconductor layer SC2 in order to block hydrogen entering thesemiconductor layer SC2.

In this variation, the block layer HB is patterned as islands, and thenumber of interfaces in the areas between thin film transistors can bereduced. Thereby, the transmissivity of the areas between the thin filmtransistors can be improved in this variation.

FIG. 7 is a cross-sectional view of a variation in which the block layeris disposed in a different position.

The insulating film 13 is disposed between the block layer HB and thesemiconductor layer SC2 in this variation, and in this respect, thevariation differs from the example of FIG. 5.

In the example depicted, the block layer HB is disposed on theinsulating film 12 and the insulating film 13 is disposed on the blocklayer HB, and the semiconductor layer SC2 is disposed on the insulatingfilm 13. That is, the insulating film 13 contacts the upper surface HBbof the block layer HB and the lower surface SC2 a of the semiconductorlayer SC2.

In this variation, the hydrogen diffusion from the insulating film 12 tothe insulating film 13 can be suppressed.

FIG. 8 is a cross-sectional view of a variation in which the block layeris patterned.

In this variation, the block layer HB is formed as islands, and in thisrespect, the variation differs from the variation of FIG. 7.

In the example depicted, the block layer HB is disposed on theinsulating film 12 and is formed as islands in the area opposed to theentire surface of the semiconductor layer SC2. The block layer HB is notformed in the other area, and is not opposed to, for example, thesemiconductor layer SC1. The insulating film 13 is disposed on theinsulating film 12 and is partly disposed on the block layer HB formedas islands. That is, the insulating film 13 partly contacts the uppersurface HBb of the block layer HB and the lower surface SC2 a of thesemiconductor layer SC2.

In this variation, the advantages obtained in the variation of FIG. 6can be achieved.

Now, an example where the semiconductor device 1 is applied to a displaydevice DSP will be explained.

FIG. 9 shows the structure of the display device including thesemiconductor device of the present embodiment. The display device DSPis, for example, a liquid crystal display device; however, it may be adifferent display device such as organic electroluminescent (EL) displaydevice.

The display panel PNL includes a semiconductor device 1. The displaypanel PNL includes a display area DA used for image display and anon-display area NDA disposed around the display area DA. The displaydevice DSP includes, in the non-display area NDA, a drive circuit Dr,signal line drive circuit SD, scan line drive circuit GD, and the like.

The display panel PNL includes a plurality of pixels PX in the displayarea DA. Note that the display panel PNL includes a plurality of scanlines G (G1 to Gn) and a plurality of signal lines S (S1 to Sm) in thedisplay area DA.

Scan lines G are drawn to the outside of the display area DA and areconnected to the scan line drive circuit GD. The scan line drive circuitGD includes complementary TFT elements CC. Signal lines S are drawn tothe outside of the display area DA and are connected to the signal linedrive circuit SD. The common electrode CE is shared by the pixels PX.The common electrode CE is drawn to the outside of the display area DAand is connected to the drive circuit Dr. Each of the drive circuits GD,SD, and Dr is used to control the electric signal supply to the displayarea DA through the scan line G, signal line S, and common electrode CE.

Each pixel PX includes, for example, a switching element SW, pixelelectrode pE, common electrode CE, and liquid crystal layer LQ. Theswitching element SW is formed of, for example, a thin film transistor.The switching element SW is electrically connected to a scan line G anda signal line S to control the luminosity of pixel PX. The pixelelectrode PE is electrically connected to the switching element SW. Thepixel electrode PE is opposed to the common electrode CE. A capacitanceCS is formed, for example, between the common electrode CE and the pixelelectrode PE.

In the example depicted, the complementary TFT element CC is formed of athin film transistor TR1 and the switching element SW is formed of athin film transistor TR2. The thin film transistor TR1 is not limited tothe complementary TFT element CC, and it may be used as a p-type TFTelement or an n-type TFT element. The thin film transistor TR1 is notlimited to the scan line drive circuit GD, and it may be included in thesignal line drive circuit SD or in the drive circuit Dr. Furthermore,the thin film transistor R1 may form a switching element SW and the thinfilm transistor TR2 may form a peripheral drive circuit such as drivecircuit Dr, signal line drive circuit SD, and scan line drive circuitGD.

As can be understood from the above, the display device DSP includes asemiconductor device 1 of the embodiment. A change in a thresholdvoltage is less in the thin film transistor TR1 than is in the thin filmtransistor TR2, for example. On the other hand, the thin film transistorTR2 has an off-current less than that of the thin film transistor TR1,for example. That is, in the display device DSP, the thin filmtransistor TR1 of high reliability can be arranged in the peripheralcircuit while the thin film transistor TR2 which can suppress leakage ofcharge can be arranged in pixels PX. Therefore, the present embodimentcan present a display device DSP of high reliability and low powerconsumption. Since the semiconductor device 1 includes thin filmtransistors TR1 and TR2 of different properties on a single substrate,the thin film transistors TR1 and TR2 can arbitrarily arranged tocorrespond to requirements of the TFTs.

As explained above, the present embodiment can achieve a semiconductordevice which can suppress degradation of reliability.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A semiconductor device comprising: an insulatingsubstrate; a first semiconductor layer formed of silicon and positionedabove the insulating substrate; a second semiconductor layer formed of ametal oxide and positioned above the first semiconductor layer; a firstinsulating film formed of a silicon nitride and positioned between thefirst semiconductor layer and the second semiconductor layer; and ablock layer positioned between the first semiconductor film and thesecond semiconductor layer, the block layer hydrogen diffusion of whichis lower than that of the first insulating film.
 2. The semiconductordevice of claim 1, wherein the second semiconductor layer contacts theupper surface of the block layer.
 3. The semiconductor device of claim2, further comprising a second insulating film formed of a silicon oxidebetween the first insulating film and the block layer, wherein thesecond insulating film contacts the lower surface of the block layer. 4.The semiconductor device of claim 1, further comprising a secondinsulating film formed of a silicon oxide between the secondsemiconductor layer and the block layer, wherein the second insulatingfilm contacts the upper surface of the block layer and the lower surfaceof the semiconductor layer.
 5. The semiconductor device of claim 1,wherein the block layer continuously extends to a position opposed tothe first semiconductor layer and a position opposed to the secondsemiconductor layer.
 6. The semiconductor device of claim 1, wherein thesecond semiconductor layer is in a position which is not opposed to thefirst semiconductor layer with the first insulating film interposedtherebetween, and the block layer continuously extends from a positionopposed to the first semiconductor layer to a position opposed to thesecond semiconductor layer.
 7. The semiconductor device of claim 6,wherein the block layer is thicker in an area opposed to the secondsemiconductor layer than is in an area opposed to the firstsemiconductor layer.
 8. The semiconductor device of claim 7, wherein theblock layer includes the upper surface of an area opposed to thesemiconductor layer which contacts the lower surface of the secondsemiconductor layer.
 9. The semiconductor device of claim 1, wherein theblock layer is formed as an island opposed to the entire surface of thesecond semiconductor layer without being opposed to the firstsemiconductor layer.
 10. The semiconductor device of claim 9, whereinthe second semiconductor layer contacts the upper surface of the blocklayer.
 11. The semiconductor device of claim 9, further comprising asecond insulating film formed of a silicon oxide between the secondsemiconductor layer and the block layer, wherein the second insulatingfilm contacts the upper surface of the block layer and the lower surfaceof the second semiconductor layer.
 12. The semiconductor device of claim1, further comprising: a first gate electrode opposed to the firstsemiconductor layer; and a second gate electrode positioned in the samelayer in which the first gate electrode is disposed and opposed to thesecond semiconductor layer, the second gate electrode formed of the samematerial used for the first gate electrode.
 13. The semiconductor deviceof claim 1, further comprising: a first gate electrode opposed to thefirst semiconductor layer; and a second gate electrode opposed to thesecond semiconductor layer and formed of the same material used for thefirst gate electrode.
 14. The semiconductor device of claim 13, furthercomprising an interlayer insulating film formed of a silicon oxidebetween the first semiconductor layer and the first insulating film,wherein the interlayer insulating film contacts the upper surface of thefirst semiconductor layer and the lower surface of the first insulatingfilm.
 15. The semiconductor device of claim 14, wherein the first gateelectrode contacts the upper surface of the interlayer insulating filmand being covered with the first insulating film, and the second gateelectrode contacts the upper surface of the interlayer insulating filmand being covered with the first insulating film.
 16. The semiconductordevice of claim 13, further comprising an interlayer insulating filmformed of a silicon oxide between the second semiconductor layer and thesecond gate electrode, wherein the interlayer insulating film contactsthe upper surface of the second semiconductor layer and the lowersurface of the second gate electrode.
 17. The semiconductor device ofclaim 1, wherein the silicon is a polycrystalline silicon.
 18. Thesemiconductor device of claim 1, wherein the metal oxide contains atleast one of indium, gallium, zinc, and tin.
 19. The semiconductordevice of claim 1, wherein the block layer is formed of aluminum oxide.20. The semiconductor device of claim 1, wherein the semiconductordevice is installed in a display panel including a display area used forimage display and a non-display area positioned around the display area,a first thin film transistor including the first semiconductor layerforms a drive circuit positioned in the non-display area to control thesupply of an electric signal to the display area, and a second thin filmtransistor including the second semiconductor layer formed a switchingelement positioned in the display area to control the brightness ofpixels.